To combat industrial pollution, it is necessary to have means to eliminate toxic wastes, including the nitrogen oxides produced in all types of combustion.
One of the standard methods in this type of purification consists in reduction of the nitrogen oxides. The reaction is performed on catalysts whose actives sites are crude platinum metals and also the metal oxides currently used for catalytic oxidation.
The reducing gas can be taken from any available sources. In the case of plants manufacturing nitric oxide by catalytic oxidation reaction of ammonia, the raw material being ammonia, ammonia is thereby the perfect source of reducing gas.
It is well known in heterogeneous catalysis that it is not sufficient to use the active elements initiating the various categories of chemical reactions; i.e., oxidation, hydrogenation, cracking, etc, but it is necessary to be able to use the most economic reaction possible.
Also, industry looks for a catalyst exhibiting the greatest possible activity, which is reflected by a slight volume of catalyst, a small size reactor and very improved conditions of the process of reduction of the nitrogen oxides, namely; low temperature, high delivery of gaseous effluents in the reactor, high rate of transformation, improved selectivity. Also a catalyst is looked for whose life is compatible with the economy of the process.
These conditions are favored, depending on the way the active sites of the solid catalyst participate in the reaction. In particular, the structure of the solid, its texture, and the promoters are, for a composition of given active elements, factors of essential importance.
Catalysts whose active sites are crude platinum metals, prepared by processes of impregnation or exchange of the platinum, have a heterogeneous structure made up of a support on which the platinum is deposited, which results in an irregular distribution of the active sites and consequently in a limited efficiency. Further, these catalysts are obtained from rare and expensive metals.
Compositions have been proposed in U.S. Pat. No. 4,071,472 that make it possible to eliminate nitrogen oxides with purification yields on the order of 99%, in reduction processes performed at high volumetric rates. These compositions contain as active elements iron, chromium and vanadium in oxide form, taken separately or in combination, in an amount between 96 and 99.5% by weight; the specific surface being between 20 and 100 m.sup.2 /g, and further the dimensions of the pores are between 100 and 5,000 A. Further, rare earth elements in an amount between 0.5 and 4% are introduced in these compositions, and at times advantageously a crude platinum metal in an amount between 0.001 and 0.0001%.
These compositions are obtained by careful decomposition of the nitrate solutions of the active elements by a controlled progression of the temperature from 200.degree. C. to 600.degree. C., by stages of 50.degree. C., maintaining each temperature stage about an hour.